The present invention relates to a method of driving a liquid crystal device comprising a liquid crystal material sandwiched between a pair of bases and, more particularly, to a method of driving a liquid crystal device comprising a pair of substrates on each of which a transparent electrode layer and an orientation films are laminated in this order. The substrates are placed opposite to each other with a given spacing between them. A ferroelectric liquid crystal material is injected into the gap.
Twisted-nematic liquid crystal devices presently available as commercial products use TFTs (thin-film transistors) forming an active matrix construction to produce a given gray scale. However, the production yield of the prior art process for fabricating these TFTs is not sufficiently high. Also, the cost is still high. For these reasons, there is a demand for a display device of larger area.
On the other hand, a display device utilizing a ferro-electric liquid crystal of surface stabilized bistable type does not need an active matrix construction consisting of TFTs or the like. Therefore, there is the possibility that an inexpensive, large-area display device is fabricated from this ferroelectric liquid crystal.
In an attempt to use ferroelectric liquid crystals as display devices, researches and developments have been vigorously made in these ten years. Generally, ferroelectric liquid crystals have the following excellent features:
(1) High-speed response. The response is 1000 times as high as the response of the prior art nematic liquid crystal display.
(2) They depend less to the viewing angle.
(3) They exhibit a memory effect.
A known technique for displaying an image on such a ferroelectric liquid crystal display is described by Clark et al. in U.S. Pat. No. 4,367,924. Specifically, the cell gap of the display panels is controlled within 2 xcexcm. The liquid crystal molecules are oriented by a restricting force which orients the molecules at the interface of the panels. The surface of this ferroelectric liquid crystal assumes only two stable energy states. Because of the response is on the order of microseconds and because of the image memory effect, researches and developments have been earnestly conducted.
In this kind of bistable mode-ferroelectric liquid crystal display, the memory effect is produced and, therefore, flicker which is a problem with a CRT can be prevented. Even in a simple XY matrix construction, the display can be driven with more than 1000 scanning lines. That is, it is not necessary to drive TFTs. Nematic liquid crystals which dominate presently have the disadvantage that the viewing angle is narrow. In contrast with this, the ferroelectric liquid crystal has a wide viewing angle because the molecular orientation is uniform and because the gap between the panels is less than half of the nematic liquid crystal panel.
Such a ferroelectric liquid crystal is constructed as schematically shown in FIG. 16. In particular, a transparent substrate 1a is made of glass. A transparent electrode layer 2a and a SiO2 oblique deposition layer 3a are successively deposited on the substrate 1a, thus forming a laminate A. The electrode layer 2a is made of ITO (indium tin oxide) that is a conductive oxide prepared by doping indium with zinc. The deposition layer 3a acts as a liquid crystal orientation film. Similarly, a transparent electrode layer 2b and a SiO2 oblique deposition layer 3b are successively deposited on a substrate 1b, thus forming a laminate B. The SiO2 oblique deposition layers 3a and 3b which are orientation films are placed in an opposite relation to each other. Spacers 4 are inserted to secure a given cell gap. In this way, a liquid crystal cell is fabricated. A ferroelectric liquid crystal material 5 is injected into the cell gap.
Although this ferroelectric liquid crystal has the excellent advantages as described above, it is difficult to realize a gray scale. In particular, the prior art ferroelectric liquid crystal utilizing bistable mode is stable only in two states and so this liquid crystal has been regarded as unsuited for creation of gray scale as required in a video tape recorder.
More specifically, when an external electric field E is applied to the prior art ferroelectric liquid crystal such as an interface stable type ferroelectric liquid crystal, the direction of orientation of molecules M is switched between state 1 and state 2, as shown in FIG. 17. The molecular orientation variations cause variations in the transmittivity if a liquid crystal display is installed between two mutually perpendicular polarizers. As a result, as shown in FIG. 18, the transmittance or transmittivity varies rapidly from 0% to 100% at a threshold voltage of Vth in the presence of the applied electric field. Generally, this voltage range in which this transmittivity makes a transition as described above is less than 1 V. Furthermore, the threshold value Vth is affected by minute variations in the cell gap. Therefore, in the prior art liquid crystal display, it is difficult to give a stable voltage range to the transmittivity-applied voltage characteristic curve. Hence, it is difficult or impossible to produce desired gray levels by controlling the voltage.
Accordingly, various methods for overcoming these difficulties have been proposed. In one method, subpixels are formed, and the pixel area is adjusted. Alternatively, pixel electrodes are divided to realize various gray levels (referred to as area gray scale method). Various gray levels are accomplished by repeating switching or line addressing in one field, by making use of high-speed switching of a ferroelectric liquid crystal (referred to as time integration gray scale method). However, these methods do not yet provide satisfactory gray scale.
In particular, in the area gray scale method, as the number of gray levels is increased, the required subpixels are increased. It is obvious that the cost performance is poor in terms of fabrication of devices and also in terms of method of driving the devices. Furthermore, where the time integration gray scale method is used alone, the practicability is low. Moreover, where the time integration gray scale method is employed in combination with the area gray scale method, the practicability is also low.
Accordingly, further methods for representing an analog gray scale in each pixel have been proposed. The distance between opposite electrodes is varied within one pixel, or the thickness of a dielectric layer formed between opposite electrodes is varied, so that a local electric field strength gradient is produced. Alternatively, the material of the opposite electrodes is varied to produce a voltage gradient.
However, complicated manufacturing steps are necessary to fabricate a liquid crystal display having analog gray level display characteristics which are at a practical level. In addition, it is very difficult to control the manufacturing conditions. Further, the manufacturing cost is high.
A still other ferroelectric liquid crystal is proposed in Japanese Patent Laid-Open No. 276126/1991. In particular, fine particles of alumina of 0.3 to 2 xcexcm are sprayed on an orientation film. The ferroelectric liquid crystal is inverted between a portion in which the fine particles exist and a portion in which the fine particles do not exist. The inversion is controlled by application of a voltage. In this way, various gray levels are produced.
Where this known technique is utilized, it is quite difficult in practice to produce desired gray levels because the fine particles are too large and because the amount of the sprayed particles is not stipulated definitely.
For example, if fine particles having grain diameter of 0.3 to 2 xcexcm is simply dispersed in a cell gap of 2 xcexcm, then it is quite difficult in practice to subtly vary inversion of the liquid crystal within one pixel. Furthermore, the ferroelectric liquid crystal produces a visible image when the liquid crystal is in a birefringent mode. This makes more difficult to control the cell gap. As a result, color nonuniformities take place. We consider that this situation is similar to the present supertwisted-nematic display in which cell gap variations are required to be suppressed within 500 xc3x85.
We have already discovered that addition of fine particles of carbon to a ferroelectric liquid crystal material can improve the electrooptical characteristics of the liquid crystal as described later. In an attempt to produce various gray levels on the ferroelectric liquid crystal having such electrooptical characteristics, a method using reset, select, and data signals to drive the liquid crystal, as illustrated in FIG. 19, has been proposed. It has been found, however, that when the liquid crystal is driven with these waveforms, the liquid crystal responds in the manner described now.
When data signal is 0, the liquid crystal molecules of the pixels to which the select signal is applied assume twisted orientation 1 for every pixel. That is, as shown in FIG. 20, the liquid crystal molecules located closest to the top substrate are oriented in a direction different to the direction of orientation of the liquid crystal molecules located closest to the bottom substrate. The direction of orientation of the molecules varies continuously in the direction of the cell. In this way, depending on the orientation, in whatever direction is the liquid crystal cell arranged between the two polarizers, it is impossible to reduce the transmittivity completely down to zero. If the data voltage increases, small domains of a second twisted orientation in which the top and bottom molecules are oriented in opposite directions are produced. As the data voltage is increased, the area of the molecules of the second twisted orientation increases. In this way, the molecules are switched to intermediate states between the two orientations, thus producing intermediate states between white and black. Hence, sufficiently high contrast cannot be accomplished.
It is an object of the present invention to provide a method adequate to drive a liquid crystal display, especially a ferroelectric liquid crystal, by a passive-matrix multiplexing drive method in such a way that an analog gray scale is realized easily and with certainty while maintaining high contrast.
The present invention resides in a method of driving a liquid crystal display comprising a pair of bases between which a liquid crystal material, especially a ferroelectric liquid crystal material is sandwiched. Let Vthlow be the voltage applied when the transmittivity of the liquid crystal material begins to change. Let Vthhigh be the voltage applied when the transmittivity of the liquid crystal material substantially assumes its maximum value. First and second select pulses of opposite polarities are applied to the liquid crystal material. Let Vs1 be the voltage of the first select pulse. Let Vs2 be the voltage of the second select voltage. This method of driving the liquid crystal display is characterized in that Vs1=xc2x1(Vthlowxe2x88x92xcex94V), where xcex94V greater than 0, and that Vs2=∓(Vthhigh+xcex94V), where xcex94V greater than 0.
In this method, the voltage range in which the transmittivity of the liquid crystal material varies can be increased, depending on xcex94V, by setting the value of the voltage Vs1 of the first select pulse and the value of the voltage Vs2 of the second select voltage toxc2x1(Vthlowxe2x88x92xcex94V) andxc2x1(Vthhigh+xcex94V), respectively. This driving method is advantageous for representation of a gray scale. Since the voltage is varied within the above-described range, even if the data pulse, or data signal, is set high, transmittivity values producing a sufficient difference can be obtained. In consequence, high contrast can be obtained between two orientation states.
In the novel liquid crystal-driving method, in order to improve the analog gray scale representation, it is desired to matrix drive a liquid crystal display having regions for switching the contained liquid crystal material. These regions have subtly different threshold values which are distributed in a range.
In order to maintain electrical neutrality of the select pulse waveform and the reset pulse waveform, to assure switching, to prevent the liquid crystal material from deteriorating, and to assure resetting, first and second reset pulses having n times (n is a real number equal to or greater than 2) as long as the pulse widths of the first and second select pulses are applied prior to the first and second select pulses. The first and second reset pulses are of opposite polarities. The first reset pulse is of the same polarity as the second select pulse. The voltage of the first reset pulse is given by Vr1=¦ (Vthhigh+xcex94Vxe2x80x2)¦, where xcex94Vxe2x80x2 greater than 0. The second reset pulse is of the same polarity as the first select pulse. Preferably, the voltage Vr2 of the second reset pulse is given by
n Vr1+Vs2=n Vr2+Vs1xe2x80x83xe2x80x83(1)
For example, 2 Vr1+Vs2=2 Vr2+Vs1.
Those voltages which are actually applied to the liquid crystal material to produce a gray scale are the first and second data pulses synchronized with the first and second select pulses. The first and second data pulses have the same pulse width as these select pulses and are of opposite polarities to these select pulses.
The novel method is adequate to drive a display device so as to produce various gray levels. This display device has glass substrates 1a and 1b having transparent electrodes 2a and 2b, respectively, as shown in FIG. 3. A material which shows optically bistable property such as a ferroelectric liquid crystal material 5 is sandwiched between the glass substrates 1a and 1b, thus forming a liquid crystal cell. In this case, in order to realize a gray scale, the use of a ferroelectric liquid crystal material containing fine particles of carbon is preferable.
The transparent electrodes include N scanning electrodes 2b extending in the X-direction on the substrate 1b and M scanning electrodes 2a extending in the Y-direction on the substrate 1a. An electrical signal for selecting and unselecting pixels is applied to the transparent electrodes 2b extending in the Y-direction. An electrical signal for displaying the contents of information, (i.e., producing black, white, and intermediate tones) is applied to the transparent electrodes 2a running in the X-direction. The device is driven by matrix multiplexing.
The liquid crystal material driven by the novel method has preferably regions having subtly different threshold values for switching the liquid crystal material. Where a black domain exists among white domains (or vice versa), the black domain is referred to as the reversed domain herein. If the transmittivity of the reversed domain is 25%, more than 300, preferably more than 600, microdomains of more than 2 xcexcm in diameter should exist in a field of view of 1 mm2. Furthermore, the threshold values in these domains should vary in a range of more than 2 V at transmittivities of 10-90%.
The liquid crystal display driven by the novel method has preferably electrooptical characteristics as shown in FIG. 4. That is, in the prior art technique, the transmittivity varies steeply, depending on the applied voltage, as shown in FIG. 18. On the other hand, in FIG. 4, the transmittivity changes relatively mildly, for the following reason. Microdomains having subtly different threshold values Vth appear in one pixel, and the transmittivity of the microdomain varies according to the magnitude of the applied voltage. In one domain, if the liquid crystal molecules show bistable property, a memory effect is produced. Still images free of flicker can be realized. Since one pixel is formed out of micrometer-order domains of different threshold values, continuously varying gray levels can be created.
The graph of FIG. 4 shows the relation of the transmittivity of a ferroelectric liquid crystal cell inserted between two mutually perpendicular polarizers to the voltage applied to the cell. The orientation of the cell is so determined that when a negative voltage exceeding the threshold value is applied to the liquid crystal cell between the polarizers, the transmittivity of the cell assumes its minimum value. The transmittivity of the liquid crystal varies continuously over a range in response to the applied voltage. Let Vthlow be the voltage at which the transmittivity of the liquid crystal material begins to vary. Let Vthhigh be the voltage at which the transmittivity of the liquid crystal material assumes its maximum value. The following relation is obtained:
Vthhighxe2x88x92Vthlow greater than 0
In FIG. 4, different threshold values result in different transmittivities. Of these different threshold values, let Vth1 be the threshold value obtained when the transmittivity is 10%. Let Vth2 be the threshold value obtained when the transmittivity is 90%. In this case, the range in which the threshold voltage varies (xcex94Vth=Vth2xe2x88x92Vth1) is preferably in excess of 2 V.
With respect to microdomains, as shown in FIG. 5(A), microdomains MD having diameters exceeding 2 xcexcm exist preferably at a density of more than 300/mm2 when the transmittivity is 25%. These microdomains transmit light at subtly varying transmittivities. As a whole, intermediate tones can be accomplished. Since this microdomain structure assumes an aspect resembling starlight, this structure is hereinafter referred to as the xe2x80x9cstarlight texturexe2x80x9d.
Because of this starlight texture the microdomains MD can be enlarged (i.e., the transmittivity is increased) as indicated by the dot-and-dash lines in FIG. 5(A) or diminished (i.e., the transmittivity is reduced), according to the amplitude of the applied voltage. Hence, the transmittivity can be varied arbitrarily, depending on the applied voltage. On the other hand, in the prior art structure, the threshold values are distributed in a quite narrow range as shown in FIG. 5(B). Therefore, in response to the applied voltage, those portions D which transmit light enlarge suddenly or disappear. Consequently, it is quite difficult to create various gray levels.
In the present invention, as a means for forming the microdomains, ultrafine particles such as fine particles of carbon can be dispersed in a liquid crystal material. FIG. 6 shows a ferroelectric liquid crystal display in which such ultrafine particles 10 are dispersed. This structure is essentially the same as the structure shown in FIG. 15.
The principle on which the threshold value is varied by the ultrafine particles 10 is now described by referring to FIG. 7. Let d2 be the diameter of the ultrafine particles 10. Let xcex52 be the dielectric constant of these particles. Let d1 be the thickness of the liquid crystal material 5 excluding the ultrafine particles. Let xcex51 be the dielectric constant of the liquid crystal material. The electric field Eeff acting on the ultrafine particles is given by
Eeff=(xcex52/(xcex51d2+xcex52d1))xc3x97Vgapxe2x80x83xe2x80x83(2)
Therefore, if the ultrafine particles having a dielectric constant smaller than that of the liquid crystal material are added (xcex52 less than xcex51), it follows that the fine particles (d2) smaller than the total thickness dgap (=d1+d2) of the liquid crystal material layer are entered. Thus,
Eeff less than Egap
As a result, a weaker electric field Eeff acts on the liquid crystal material than the field (Egap) acting when no ultrafine particles are entered. Conversely, if fine particles having a dielectric constant larger than that of the liquid crystal material is added (xcex52 greater than xcex51), the relation given by
Eeff greater than Egap
is obtained. In consequence, a stronger electric field Eeff acts on the liquid crystal material than the field Egap acting when no fine particles are entered.
In summary, the following relations hold:
When xcex51 greater than xcex52, Eeff less than Vgap/(d1+d2)=Vgap/dgap=Egapxe2x80x83xe2x80x83(1)
When xcex51=xcex52, Eeff=Egapxe2x80x83xe2x80x83(2)
When xcex51 less than xcex52, Eeff greater than Egapxe2x80x83xe2x80x83(3)
In any case, the effective electric field Eeff applied to the liquid crystal material itself is varied by the addition of the ultrafine particles. It follows that the effective electric field applied to those regions of the liquid crystal material in which the ultrafine particles exist is different from the effective field applied to those portions of the liquid crystal material in which the ultrafine particles do not exist. As a result, even if the same electric field Egap is made to act on the liquid crystal material, reversed domains appear in some regions but do not in the other regions. In this way, the starlight texture structure as shown in FIG. 5(A) can be developed.
The above-described starlight texture structure is adapted for creation of continuously varying gray levels. Various transmittivities (i.e., two or more gray levels) can be obtained by controlling the applied voltage (i.e., its amplitude, pulse width, or the like) under the presence of ultrafine particles. On the other hand, where fine particles are simply added as in the prior art techniques, only the structure shown in FIG. 5(B) is derived. Obviously, even if fine particles of 0.3 to 2 xcexcm are dispersed in a minute gap (on the order of 2 xcexcm), the desired display characteristics are not obtained. Apart from the minute gap, fine particles cause unwanted color nonuniformities, which will be described in detail in Comparative Example. In the present invention, such an undesirable phenomenon does not take place. Rather, the desired performance can be obtained.
In the novel liquid crystal display, the fine particles added to the liquid crystal material should cause the effective electric field strength applied to the liquid crystal material 5 to be distributed over a range, the liquid crystal material 5 existing between opposite transparent electrode layers 2a and 2b shown in FIG. 6. For example, fine particles of plural kinds having different dielectric constants may be mixed and used. The presence of fine particles having the different dielectric constants produces a distribution of dielectric constant within each pixel. As a result, if a uniform external electric field is applied between the transparent electrode layers 2a and 2b of the pixels, the effective electric field intensity applied to the liquid crystal material in the pixel has a distribution. This increases the range of the threshold voltage for switching the liquid crystal material, especially the ferroelectric liquid crystal material. Hence, an analog gray scale can be accomplished within one pixel.
Where fine particles of the same dielectric constant are used, the sizes of the particles should have a distribution. In this way, the thickness of the liquid crystal material layer is made to have a distribution by the presence of the fine particles which are not different in dielectric constant but differ in size. As a result, even if a uniform external electric field is applied between the transparent electrode layers 2a and 2b within one pixel, the effective electric field strength applied to the liquid crystal material within the pixel exhibits a distribution. This permits creation of an analog gray scale. If the sizes of the fine particles are distributed over a considerably wide range, an excellent analog gray scale can be accomplished.
In a liquid crystal display for use in the present invention, fine particles added to the liquid crystal material have surfaces preferably having pH of more than 2.0 because if the pH is less than 2.0, the acidity is too strong. In this case, the liquid crystal material is readily deteriorated by protons.
Preferably, the fine particles added to the liquid crystal material is less than 50% by weight and higher than 0.1% by weight. If the amount of addition is too large, the particles coagulate, thus making it difficult to develop the starlight texture structure. Also, it is often difficult to inject the liquid crystal material.
The usable fine particles can be carbon black and/or titanium oxide. In one example, the carbon black is carbon black fabricated by the Farness"" process, and the titanium oxide is amorphous titanium oxide. The grain sizes of the fine particles of carbon black produced by the Farness"" process have a relatively wide distribution. The amorphous titanium oxide has good surface properties and excellent durability.
Used fine particles preferably have sizes which are half (less than 0.4 xcexcm, more preferably, less than 0.1 xcexcm) of the liquid crystal cell gap when they are not coagulated, i.e., in a primary fine particle state. The gray level display characteristics can be controlled by the grain size distribution. Where the standard deviation of the grain size distribution is more than 9.0 nm, the variations in the transmittivity or transmittance can be made milder with desirable results. If the specific gravity of the fine particles is 0.1 to 10 times the specific gravity of the liquid crystal material, the fine particles are prevented from settling when they are dispersed in the liquid crystal material. If the surfaces of the fine particles are processed with silane coupling agent or the like, good dispersion is obtained.
In the present example, it is necessary that the fine particles exist between the electrodes 2a and 2b opposite to each other. No restrictions are imposed on the locations in which the fine particles are placed. The particles may be positioned within the liquid crystal material 5, within the orientation films 3a and 3b, or on these films 3a and 3b. 
A liquid crystal display used in the present invention can be fabricated by an ordinary method. For example, a transparent ITO film is formed on a glass substrate by sputtering technique. The film is patterned photolithographically. Then, SiO is obliquely deposited on the substrate by vacuum evaporation. After a liquid crystal cell is assembled, a liquid crystal material in which fine particles are uniformly dispersed is injected into the cell gap. In this manner, the liquid crystal display is fabricated. A rubbed film of polyimide or a film on which SiO is deposited obliquely can be used as a liquid orientation film.
Where the orientation film is a film formed by depositing silicon oxide, the layer is annealed after the deposition. The surface property is varied, so that a starlight texture structure is developed.
The present invention is well suited for the above-described starlight texture structure to which fine particles are added. The invention can also be applied to an ordinary liquid crystal structure not having the starlight texture structure.
That is, the invention improves the driving waveforms used in a liquid crystal display (especially, an inexpensive, large area liquid crystal display driven by passive matrix addressing without needing TFTs or the like) capable of developing the above-described starlight texture structure. The driving waveforms are used to cause the liquid crystal material to produce intermediate tones. The transmittivity of the liquid crystal material varies continuously over a certain range in response to an applied voltage.
One driving waveform used in the novel method are an electrical select signal (scanning waveform) applied to the scanning electrodes 2b formed on the substrate 1b, as shown in FIG. 1(a). The scanning electrodes 2b extend along the Y-direction. This waveform has the following features.
(1) The select signal is composed of two kinds of pulses, or positive pulses Vs1 and negative pulses Vs2. As shown in FIG. 4, the threshold voltage of the transmittivity variation (Tr)-applied voltage (V) curve of the liquid crystal cell between the two mutually crossing polarizers is Vthlow. The select pulse voltage is determined by the threshold value of the liquid crystal display. The pulse width is determined by the response speed of the liquid crystal material.
The height of the positive select pulses Vs1 is the difference between the voltage Vthlow at which starlight texture appear on the monodomain structure of the liquid crystal material displaying black and xcex94V, i.e., Vthlowxe2x88x92xcex94V. The height of the negative select pulses Vs2 is the sum of the voltage Vthhigh at which the liquid crystal state is completely switched to a state in which white is displayed and xcex94V, i.e., xe2x88x92(Vthhigh+xcex94V). xcex94V is a positive voltage. To produce various gray levels, the voltage xcex94V is required to have a larger value but it is restricted by the voltage of the driver circuit. This voltage xcex94V increases the range of the threshold value. This is quite advantageous to obtain a gray scale.
(2) Two reset pulses Vr1 and Vr2 are applied before the select pulses Vs1 and Vs2. These reset pulses are n times as wide as the select pulses. For instance, the width of the reset pulses is two times the width of the select pulses. The voltage of the reset pulses is determined according to the following relations. The first reset pulses Vr1 are of the same polarity as the second reset pulses Vs2. The second reset pulses Vr2 are of the same polarity as the first select pulses Vs1. The first reset pulses Vr1 act to completely switch the present display state of the liquid crystal material to another state. The voltage of the first reset pulses Vr1 is the sum of the Vthhigh and a low voltage xcex94Vxe2x80x2. This low voltage xcex94Vxe2x80x2 assures resetting of the liquid crystal material. The voltage Vr2 of the second reset pulses is determined according to the following equation.
xe2x80x83n Vr1+Vs2=n Vr2+Vs1xe2x80x83xe2x80x83(1)
where n is a real number equal to or greater 2. Normally, n is 2 to 4, preferably about 2. For example, 2 Vr1+Vs2=2 Vr2+Vs1; Vr2 greater than Vthhigh. In the Eq. (1) above, Vr1, Vr2, Vs1, and Vs2 are the voltages of the first reset pulses, the second reset pulses, the first select pulses, and the second select pulses, respectively.
The condition given by the Eq. (1) above is used to maintain electrical neutrality of the select waveforms and of the reset waveforms. When a dc electric field is applied to a liquid crystal material, it induces an electrode reaction or electrode process on the surface of the orientation film. As a result, electric charges tend to be accumulated on one electrode. This degrades the liquid crystal material. The electric charges are neutralized by setting the pulse voltages under the condition of Eq. (1) above. Hence, the liquid crystal material is prevented from deteriorating.
An electrical data signal applied to the data electrodes 2a which are formed on the substrate 1a in the X-direction is shown in FIG. 1(b). This waveform has the following features:
(1) The electrical data signal is composed of a negative pulse VD1 and a positive pulse VD2 whose waveforms are symmetrical. These pulses have the same width as the select pulses Vs1 and Vs2. The height VD of the data voltage varies from 0 to Vthhighxe2x88x92Vthlow according to the gray level to be displayed on the liquid crystal display.
(2) The voltage pulses VD1 and VD2 are of opposite polarity to the select pulses Vs1 and Vs2. In this way, the voltage applied to the pixel at the address (n, m) on the display is the sum Vs+VD, which is shown in FIG. 1(c).
With respect to pulse widths, it is assumed that the reset pulse width is equal to the select pulse width, or the data pulse width, as shown in FIG. 2. When the phase of one data pulse is reversed and applied as indicated by the broken lines, the reset pulses Vr1 and Vr2 are reduced to levels Vr1xe2x80x2 and Vr2xe2x80x2, respectively, by an amount corresponding to the data pulse. As a result, resetting cannot be effected. However, the reset pulse width is set to a value that is n times the select pulse width, or the data pulse width. Therefore, even if the phase of the data pulse is reversed, it is assured that sufficient reset pulse voltage (xe2x89xa7Vr1, Vr2) is obtained. Hence, resetting can be carried out with certainty at all times.
The driving method using the above-described driving waveforms is summarized as follows (see FIG. 1(c)).
(1) At voltage V1, the presently displayed gray level is once reset completely to white state. Since the voltages V1 and V4 are of the same polarity, the currently displayed level is momentarily brought to white level.
(2) At voltage V2, the liquid crystal material reset to white level is reset completely back to black level, thus making preparations for the next writing.
(3) Whatever data voltage is applied, voltage V3 is lower than the voltage Vthlow. Therefore, the ferroelectric liquid crystal material does not respond at this time. However, since the voltages V3 and V2 are of the same polarity, the sum of these two voltages acts on the ferroelectric liquid crystal material. In any case, transmission is not affected, because black level is displayed.
(4) Voltage V4 controls the gray level to be displayed next. The displayed gray level varies according to the area or amplitude of the voltage V4.
Other objects and features of the invention will appear in the course of the description thereof, which follows.